[Completed] HP 6825A "Bipolar Power Supply / Amplifier" restoration

[Haatveit]

Picked this thing up today. It looks a little worse for wear, but more critically, it *feels* terrible! It's completely covered in some sticky, dusty, oily substance. Not sure if owned by a chain smoker or used in some dirty environment, but this thing needs a thorough restoration! Inside and outside. I was told it was working, but I have no idea how they verified that, considering the control knobs, for the most part, are so gummed up that you either can't feel the detents, or, can't move the knobs at all! At least, I don't dare put enough force on them to turn them in their current state.

This thing is really heavy. Not sure why they need that huge massive transformer, but, there it is! Lovely vintage HP construction. Plug-in-boards all around, over engineered to the hilt. That's why I love these things! I have an old 5215A frequency meter, also a beauty with lots of individual plug in boards and loomed wiring. Same style outer shell as this one.

Hope to get this one restore to mint, or close to it. I do use these instruments, but they are in large part also a collection, so I really like getting them to a pristine condition. The "physical" condition of this one is very good which is why I bought it. No big dents or scratches. Very dirty, but, dirt can be removed!

Just started taking the panels off and washing them. Will post more as I go deeper....


I haven't started to dig into the service manual yet, but, here it is. I didn't even know these types of instruments existed until I stumbled over them on eBay. They seem quite rare. Kepco seems to be the biggest, and most current, manufacturer, and they call them "Operational Power Supplies" - very neat units, there's a bunch of Kepco units on eBay. Although they are quite expensive, and usually very large and heavy, so cost a fortune to get shipped.

They don't go up to very high frequencies if used as amplifiers, but, they are rather neat regardless. And there's always room for a bipolar power supply or two on the bench...

[Haatveit]

Alright, broken it down into quite a few parts so far. Everything is sticky!!

Been methodically cleaning everything as I take it apart. Fortunately, beside the sticky issue, everything looks really good. I think this should clean up quite well.
The thermal paste under the big TO-9 trannies has basically turned into a fine, dry, powder - so I took them off (they were thankfully socketed on a separate little daughterboard), and am gonna give them a good clean up, and apply some new thermal paste.

Also in the pictures is the very nerdy voltage selection mechanism - a tiny little PCB gold plated pads on them. You take the little PCB out (which is only possible while the line input is covered with a plastic slide door), and turn it so that the line voltage you require is readable, i.e. not upside down. Then you put the little PCB back into it's slot, and off you go.
My problem to tackle tomorrow is the transformer. I want to get it out, but it looks like a pretty involved task. Might just de-solder the connections, and rip it out, so that everything is accessible. But that's quite a lot of work!

[bitseeker]

I've encountered several devices in that style of case that were sticky all over, too. I'm not sure what it is, either, but it doesn't smell like cigarette smoke residue. It's really hard to remove as I wasn't comfortable using anything too harsh lest it damage the vinyl coating on the panels or the lettering. I tried alternating among household degreaser (Simple Green), isopropyl alcohol, adhesive remover (Goo Gone), and soapy water. Eventually, most of it came off. Nevertheless, I did end up wiping some lettering off the rear panel of an old DMM that had that gunk on it.

As for the huge transformer, it's not unusual considering that this is a linear power supply/amplifier that has to be able to deliver -20 to 20V at 2A.

[Haatveit]

I've encountered several devices in that style of case that were sticky all over, too. I'm not sure what it is, either, but it doesn't smell like cigarette smoke residue. It's really hard to remove as I wasn't comfortable using anything too harsh lest it damage the vinyl coating on the panels or the lettering. I tried alternating among household degreaser (Simple Green), isopropyl alcohol, adhesive remover (Goo Gone), and soapy water. Eventually, most of it came off.

Unfortunately it's not the same issue here. My old frequency meter had the sticky vinyl covering, dish detergent did a nice job with it - but this power supply has a serious problem. It's so insanely sticky that the knobs on the front panel are completely seized up - one of them cannot be turned at all, even with pliers I'm only able to slowly rotate it, and I'm putting so much force on it I'm afraid something is just going to break. And the others move very slowly, as if they've been caked in 100 year old syrup... Really icky stuff, whatever this is. And it's everywhere.

[Haatveit]

After much cleaning of side panels and other bits and bobs, I've come to the conclusion that the only way I can get to the rest of the machine to do whatever needs to be done, is to desolder the ganged rotary switches. I thought I would have to desolder the transformer from the bottom board, but thankfully seems like I can dodge that one. I started to tear the front panel down, and it looks like I can get just enough access to get to everything from here!

Going to desolder the meters as well, just so I don't damage them while handling the unit during the rest of this restoration... Which is turning out to be a bigger job than I thought!

Also some pictures of the big huge General Electric caps in the unit. They were actually screwed into the PCB, no soldering at all. Interesting, and very easy to remove! They measure OK on the capacitance mode on my DMM, but I don't really know how they're doing leakage wise, or what their original spec was in the first place.

[bitseeker]

Regarding the caps, if you don't have access to an ESR meter, when you get the supply back together enough to run it on the bench, check how much ripple is on the output. If it's within spec, the caps are likely OK. They don't get hot like caps in an SMPS and can last a long time. I have some power supplies from the 60's and their filter caps are still A-OK.

[Haatveit]

Aha, fixed the knobs! They shaft running inside the brass tubes were seized with old grease or something. Couldn't be turned. So I removed and disassembled the knob assemblies, cleaned them very thoroughly and now they feel like new. Kind of funny how so little could cause such a complete failure of the mechanism - I've lubricated the actual shaft with a little bit of machine oil from my fingers, hopefully that should keep it going nice and smooth for years to come. Should probably have used some kind of thin grease, but I didn't have anything on hand that I knew the long term characteristics of...

So, now the knobs have very nice, distinct snap positions. Very interesting wafer construction, I'm guessing these could be stacked and configured very easily to any desirable configuration.

I feel like I'm getting close to the end! All I need to do now is some final cleaning, re-attaching the knobs to the board, and putting all the thousand pieces I have scattered throughout my table back into something that hopefully resembles a 6825A.

Then, finally, comes the fun part of actually turning it on (for the first time) and calibrating it, and doing some tests.

[bitseeker]

Good stuff! Looking forward to seeing it up and running.

[SeanB]

Those GE units will be fine, so long as the little rubber bung on the top is not cracked and thus they are dry. Took a 40 year old NOS 10 000uF 6V3 unit, that had not been powered for a decade or three, measured capacitance and ESR, then charged it up to 5V via a resistor and then left it open circuit for a week, and it held the charge with little droop. The ones I would worry about are the orange Sprague units, they have a nasty habit of evaporating electrolyte through that rubber fronted SRBP cap and having a high ESR with time, if your capacitance check shows they have less than 80% of the rated capacitance they need changing.

[Haatveit]

Those GE units will be fine, so long as the little rubber bung on the top is not cracked and thus they are dry. Took a 40 year old NOS 10 000uF 6V3 unit, that had not been powered for a decade or three, measured capacitance and ESR, then charged it up to 5V via a resistor and then left it open circuit for a week, and it held the charge with little droop. The ones I would worry about are the orange Sprague units, they have a nasty habit of evaporating electrolyte through that rubber fronted SRBP cap and having a high ESR with time, if your capacitance check shows they have less than 80% of the rated capacitance they need changing.

Thanks. As it turns out, there were more problems and you were spot on about the orange Sprague caps, along with several others. This put a bit of a spanner in the excitement as I wasn't really planning on doing a full re-cap of this device, but there was so much ripple on the power rails... As in, many volts of ripple. And after desoldering the orange caps (and noticing the fact that one end of each cap had kind of blown out, hard to tell exactly what has happened) I can confirm they are no longer capacitors!

I believe the GE ones are probably fine, but I got some replacement caps since I have to pay shipping anyway. We'll see if I end up using them or not. I think there may be more troubles with this unit, but for now I have to wait and see, it's hard to troubleshoot more with a DC rail that looks like an AC rail.

Oh well, at least for now, it looks good next to my 5210A (which is in great working condition, I calibrated it the other day)!

[Haatveit]

A little package full of big caps from DigiKey just arrived. Time for a re-cap of this thing, and hopefully it works now, or, at the very least, I can start trouble shooting it properly!

Seems I ordered one of the caps wrong, I picked up some 2200uF radials instead of ~300uF .... But I don't think it matters, looking at the schematics, they are just filter / bulk caps for the supplies, so shouldn't cause any issue, the new voltage rating exceeds the old ones.

[bitseeker]

The only potential issue that comes to mind with larger bypass caps is the increased inrush current. It may be just fine, but given that it's 7x more capacitance, it's something to check for.

[Haatveit]

The only potential issue that comes to mind with larger bypass caps is the increased inrush current. It may be just fine, but given that it's 7x more capacitance, it's something to check for.

I thought about that - but it ought to be fine, the current path to the caps in question is fairly straight forward, no pass transistors or anything of the kind in front of them. There are some diodes, but I feel like they should be fine for such a short increased peak load. If not, I'll find out...

[bitseeker]

Yep. Onward with the restoration.

[Haatveit]

And we're done!

Image gallery here (Imgur)

Turns out I had re-assembled one of the wafer switches a little wrong, my earlier conclusion that they were symmetrical was wrong... It was the end of a long day when I made that conclusion But no biggie, I quickly figured it out and fixed it. The capacitors went in just fine, needed a little jumper link on one of the replaced "Twist Lock" caps, since they had utilized more than 1 ground lead.

The only thing I'm not happy with is my attempt at replacing a broken line indicator bulb on the front - it used to be an incadescent bulb connected to 42V. I couldn't find a replacement on short notice, so I bodged in an LED with current limiting resistor... Had to settle for really low current due to the voltage drop. Turned out very dim, not happy with it. But it'll do until I find a proper replacement. It's very cramped in there, so it has to be tiny!

Anyhow, finally I powered it on and did some very basic functionality checks, it supplied power, limited current, range switches worked fine, meters worked fine.

I went ahead and performed a small subset of the calibration / adjustment routines in the service manual, but I lacked some power resistors and other stuff to do a full adjustment. I'll follow up with that later. It seemed pretty bang on regardless 

Finally, I set up my function gen and scope, and did some playing around with amplifier modes! That's what I find so cool about this device - not only is it a cute little bipolar power supply, it's an amplifier! I've seen these things described as Operational Power Supplies and it's a fitting name. Also very handy in that it can sink current as well, up to 50% of it's rated source current over the entire voltage span. So it's a low power electronic load, even programmable if you want to mess with the remote programming terminals on the back!

As an amplifier, the spec claims a 3db knee at ~40KHz in Fixed 1x Gain mode, or at ~15KHz in variable gain mode. Pretty ordinary, but very cool considering what it is. I did some testing (see linked gallery) and it seemed quite happy to do square waves up to about 40KHz where the rising edge started to round off, but still usable. Sine waves looked great all the way up above 100KHz, but the voltage drop was quite significant at that point. Beyond that, it started to consume quite a lot of current just to keep up with the slew rate. This was with no load.

Restoration complete! 

[bitseeker]

Nice job. Looks great! I didn't know these amplifiers could sink, too. I've contemplated getting one for some time. Now, I want one even more.

Thanks for the restoration tour.

[markav2]

I picked up an HP 6825A a couple days ago at auction for $18.50. Was not listed as nonworking so hopefully, it is but if not thank you for sharing your experiences.

[bitseeker]

$18.50! Congrats on that score. Even if it's broken, it was worth it.